Chapter 11:
Salts
What are salts?
A salt is a compound formed when
a metallic ion or an ammonium ion (NH4+)
replaces one or more hydrogen ions of an acid.
Example:
Zinc sulphate is the salt formed
when the zinc ion replaces the hydrogen ion
of sulphuric acid.
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Formation of Zinc Sulphate
zinc hydroxide + sulphuric acid
 zinc sulphate + water
Zn2+ comes from the base, Zn(OH)2
Zn(OH)2(s) + H2SO4(aq)  ZnSO4(aq) + 2H2O(l)
SO42– comes from sulphuric acid, H2SO4
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Examples of salts and reactants used
to make them
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Water of Crystallisation
Many salts combine with water molecules to form
crystals. These water molecules in the crystals
are known as water of crystallisation.
Salts that contain water of crystallisation are
called hydrated salts.
Salts that do not contain water of crystallisation
are called anhydrous salts.
Anhydrous salts are often powders.
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Water of Crystallisation
The dot ‘.’ in a
formula means that
if the substance is
heated, everything
after the dot will be
given off first.
.
ZnSO4 7H2O
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For hydrated salts,
water will be given
off first on heating.
When a hydrated salt is heated, water of
crystallisation is given off. For copper(II) sulphate,
there is a colour change.
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List of Soluble and Insoluble Salts
Any salt that
It is essential
contains
to memorise
sodium,
this table.
potassium,
ammonium or
nitrate is
always soluble.
Examples:
NaCl, K2SO4,
NH4Cl
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Soluble and Insoluble Salts
Besides AgCl
and PbCl2, all
chloride salts
are soluble.
Examples:
NaCl, FeCl3,
ZnCl2
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Soluble and Insoluble Salts
Besides
BaSO4, CaSO4
and PbSO4, all
sulphate salts
are soluble.
Examples:
Na2SO4,
Fe(SO4)3,
ZnSO4
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Soluble and Insoluble Salts
Besides these
3 carbonates,
all carbonates
are insoluble.
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Preparing Salts
There are a few ways to make salts.
Before deciding on how to prepare a salt, you must
consider two factors:
1. Is the salt soluble in water?
2. Are the starting materials soluble in water?
Soluble salts are
prepared by reactions
with acids.
Insoluble salts are
prepared by
precipitation reactions.
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(Refer to the TB at the same time)
First, decide
whether a salt is
soluble or not.
If the salt is soluble
in water, we react an
acid with a suitable
metal, base or
carbonate to get the
salt.
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Next, decide between
these 2 methods:
a) reaction with an
insoluble substance
or
b) titration.
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You will now learn how to prepare the following
soluble salts by reaction with acids:
• zinc sulphate,
• copper(II) sulphate,
• magnesium chloride and
• sodium nitrate
and the following insoluble salt by precipitation:
• lead(II) sulphate.
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Method 1: Reacting an Acid with an
Insoluble Metal, Base or Carbonate
1. We react the acid
with an excess of the
substance (MBC).
2. The substance must
also be insoluble
in water.
Reason:
This ensures that all the
acid is used up.
Reason:
This allows the excess
substance to be filtered
from the salt solution
produced.
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How can we prepare the salt ZnSO4
by reacting an acid with a metal?
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Making zinc sulphate crystals
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Making zinc sulphate crystals
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Why is this method NOT suitable for
some metals?
P
S
C
It is not suitable for reactive metals such as
potassium, sodium and calcium. They react
violently with acids
M
A
Z
I
L
This method is suitable for moderately
reactive metals such as zinc, magnesium,
aluminium and iron.
C
S
It is not suitable for unreactive metals such as copper
and silver. These metals do not react with dilute acids.
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How can we prepare a salt by
reacting an acid with a base?
A salt can also be prepared by the reaction between
an acid and an insoluble base (metal oxide or metal
hydroxide).
For example, to produce copper(II) sulphate, we react
dilute sulphuric acid with copper(II) oxide.
copper(II) oxide + sulphuric acid
 copper(II) sulphate + water
CuO(s) + H2SO4(aq)  CuSO4(aq) + H2O(l)
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How can we prepare a salt by
reacting an acid with a carbonate?
We can also prepare soluble salts from insoluble
carbonates. The steps are the same as those
described for metal and bases.
2 major differences compared with reacting with bases:
1. No heating is required.
2. There is effervescence, so we can easily
tell when all the acid has been used up.
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How can we prepare MgCl2 by
reacting an acid with a carbonate?
Magnesium chloride can be made by reacting
dilute hydrochloric acid with magnesium
carbonate.
MgCO3(s) + 2HCl(aq)
 MgCl2(aq) + CO2(g) + H2O(l)
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Method 2: Titration
In titration, we often react an acid with a
soluble substance (e.g. sodium hydroxide
or sodium carbonate).
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Comparison between Method 1 and
Titration (method 2)
Method 1
Uses one insoluble
substance.
We can tell when all the
acid is used up. When
all the acid is used up,
we see a residue of the
insoluble metal, base or
carbonate.
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Titration
Uses two soluble
substances.
No residue seen when
reaction completes.
How can we tell a reaction is
complete in titration?
To determine whether a reaction is complete,
we use an indicator.
Sodium, potassium and ammonium salts are
prepared using the titration method.
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When do we use titration?
Sodium, potassium and ammonium salts
are prepared using the titration method.
Examples: NaCl, K2SO4, NH4Cl
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Preparation of Sodium Nitrate by
Titration
The preparation of sodium nitrate involves
a) titration to determine the volumes of
reactants required,
b) actual preparation of sodium nitrate.
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Preparation of Sodium Nitrate by
Titration
This part is titration.
Next, we will study how
titration is performed to
determine volumes.
This part is the
preparation of the salt.
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Titration (to prepare NaNO3)
1. Fill up a burette with dilute nitric
acid. Note the initial burette
reading (V1 cm3).
2. Pipette 25.0 cm3 of dilute
sodium hydroxide solution into a
conical flask.
3. Add 1 or 2 drops of methyl
orange (indicator) to the
sodium hydroxide solution.
The solution turns yellow.
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Titration (to prepare NaNO3)
4. Add dilute nitric acid from
the burette slowly until the
solution just turns orange
permanently (no longer
yellow).
This is the end-point.
5. Record the final burette
reading (V2 cm3).
Hence, the volume of acid required
for complete neutralisation
= (V2 – V1) cm3.
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Preparation of Sodium Nitrate after
Titration
1. Pipette 25.0 cm3 of sodium hydroxide
solution into a beaker.
Do not add the
indicator, as it will
2. Add (V2 – V1) cm3 of dilute nitric acid
make the salt impure.
from the burette.
3. Heat the solution to evaporate the water until it is saturated.
4. Allow the saturated solution to cool and crystallise.
5. Filter to collect the crystals.
6. Dry the crystals between sheets of filter paper.
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Detecting End-point Using
a pH Meter
The pH of the solution in the conical flask changes
during an acid-base titration as solution is added from
the burette.
These changes in the pH values can be measured using
a pH meter.
Hence, a pH meter can also be used to determine the
end-point of a titration.
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Detecting End-point Using a pH Meter
We titrate 1.0 mol/dm3 sodium hydroxide solution with a
1.0 mol/dm3 nitric acid. The pH curve is shown here.
The pH decreases from 14 as
more nitric acid is added.
At the end-point, there is
sharp drop in pH.
It finally ends near pH 0.
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Method 3: Precipitation
– to produce insoluble salts
All insoluble salts can be prepared by precipitation.
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Method 3: Precipitation
To precipitate an insoluble salt, mix 2 solutions together.
One solution contains the positive ions of the salt.
The other solution contains the negative ions of the salt to be prepared.
For example, insoluble lead(II) sulphate
can be prepared by
using a soluble lead(II) salt
(such as lead(II) nitrate)
+
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dilute sulphuric acid or
any soluble sulphate.
Method 3: Precipitation - Steps
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Identifying Gases
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Identifying Gases
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Qualitative Analysis
All salts are ionic compounds. Hence, they
contain cations (positive ions) and anions
(negative ions). We can make use of this idea to
identify an unknown salt.
The process of identification of cations and
anions is known as qualitative analysis.
Tip : In order to identify an unknown salt,
it must be dissolved in water.
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Identifying Cations
Most cations give precipitates with alkalis.
Exceptions: Na+, K+ and NH4+
A cation can be identified by noting the following
a) the colour of the precipitate produced,
b) whether the precipitate is soluble or insoluble
in an excess of an alkali,
c) whether ammonia gas is liberated on addition
of sodium hydroxide solution.
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What are the precipitates?
The precipitate in each of the reactions is the
hydroxide of the metal ion.
For example, the reddish-brown precipitate in
a solution containing iron(III) ions is iron(III)
hydroxide, Fe(OH)3.
Fe3+(aq) + 3OH-(aq)  Fe(OH)3
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What are the precipitates?
Some of the precipitates dissolve in excess
sodium hydroxide or aqueous ammonia
(for example, solutions containing zinc ions or
copper(II) ions).
This is due to the formation of compounds that
are soluble in water.
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Identifying Anions
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Identifying Gases
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Identifying Gases
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How do we test for the presence of
water?
When a substance is heated, a colourless
liquid that condenses near the top of the test
tube is most likely to be water given off by a
hydrated salt.
There are two chemical tests that will show
whether a liquid is water.
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Identifying Water
1. Test with anhydrous copper(II) sulphate
Water will change the colour of anhydrous
copper(II) sulphate from white to blue.
2. Test with anhydrous cobalt(II) chloride
Water will change the colour of dry
cobalt(II) chloride paper from blue to pink.
Note that these two tests only show the presence of water.
They cannot be used to test for the purity of water.
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